This invention relates to a method and apparatus for correcting the output signal from a facsimile scanner and more particularly to an apparatus for cancelling the nontonal variations in the output signal of a facsimile scanner introduced by the scanning mechanism so as to provide a signal accurately representative of the tonal level of the scanned document.
Facsimile scanners progressively scan over the surface of a document successively measuring the contrast or tonal level of small discrete areas of the document and generate a signal related to the tonal level of each area of the document measured. One arrangement for accomplishing such scanning is described in U.S. patent application No. 621,902 filed Oct. 14, 1975, entitled "Apparatus and Method for Facsimile Scanning" by J. S. Abrams and having a common assignee with this invention. As described in this reference, light is reflected sequentially from each document area as determined by the coincidence of two rotating spiral apertures and a longitudinal slit. The reflected light is in the form of a serial stream of light impulse levels with the intensity of each impulse determined by the contrast of the area sampled. The photo pulses are passed through an optical system to a photodetector which converts the light levels into electrical signals for transmission to a graphic printer.
To facilitate physical positioning of the copy in the scanner, the copy is conveniently placed on a flat, transparent plate and scanned through the plate. It will be appreciated that with such a physical arrangement the light path length from the copy to the photodetector at extreme ends of the scan line is greater than the path length at the center of the copy. Thus, a document displaying a uniform gray scale over an entire scan line produces increased signal intensity at the center of the scan as compared to the signal intensity at the ends of the scan. It has been found that, with such an arrangement, the output signal, in response to a constant copy tonal level, varies as the cosine of the acute angle which the scanning path forms with the document raised to the fourth power (cosine.sup.4).
Additionally, the illumination level from the light source over the document area may vary as a result of the unevenness and position of the light source introducing additional nontonal variations into the photodetector output signal. Thus, as a result of the characteristics of the scanning apparatus, variations may be introduced into the scanner output signal causing it to depart from a true representation of the tonal level of the document. Various systems have been suggested for compensating for such variations. One particular arrangement uses an aperture of varying size positioned between the light source and the document. The aperture serves to decrease the light intensity at the center of the document and increase the light intensity over the edges of the document. The construction of such an aperture is particularly difficult and therefore expensive since critical tolerances must be met. Additionally, the presence of such a light aperture greatly complicates the physical structure of the scanner.
Still another effort at correcting for such nontonal signal variations has been to place a varying intensity, neutral density filter between the light source and the document. Such a filter serves to reduce the light level directed at those portions of the document closest to the photodetector. The manufacture of such a filter is relatively expensive and requires the placement of a mounting structure within the facsimile scanner. The physical presence of such a filter considerably complicates the internal mechanical arrangement of the scanner and is rather expensive to manufacture. The illustrated embodiment corrects nontonal variations in the photodetector output signal by means of an electrical circuit which introduces a compensating signal serving to cancel nontonal variations in the output signal of the photodetector. As will subsequently be more fully appreciated, the illustrated circuit may be adapted to correct for nonuniform variations in the photodetector output, and since the correction apparatus is implemented by electronic components, the illustrated embodiment does not materially effect the complexity of the physical structure of the scanner.